This invention relates to coordinate position digitizing systems adapted to be employed in apparatus for displaying or creating figures. Data representing a figure drawn by a stylus on a tablet, which is disposed at a position close to a display device having a cathode ray tube, are obtained and are used for reproducing the figure thus drawn on the tablet, or for creating a required figure on the cathode ray tube, or for changing a display on the cathode ray tube.
There have been proposed several kinds of position determining system which are used in apparatus for displaying or creating figures containing a picture display device having a cathode ray tube, and each of which comprises a position determining plate forming a tablet disposed at a position close to the cathode ray tube and a stylus used for drawing figures on the position determining plate. In one of such position determining systems, a coordinate position digitizing system in which the position of the stylus on the position determining plate is magnetically detected has been already proposed by the assignee hereof.
In the coordinate position digitizing system proposed by the assignee, the position determining plate forming the tablet is provided thereon with a plurality of conductors extending parallel to one another at regularly spaced intervals to be supplied with a constant current during a constant short period one after another, and the stylus comprises magnetic flux detecting means operative to convert the magnetic flux produced in response to the current flowing through the conductors on the position determining plate into an electric signal, and the position of the magnetic flux detecting means forming the stylus on the position determining plate in the direction of arrangement of the conductors is detected in response to the signal from the magnetic flux detecting means.
The magnetic flux detecting means is embodied with a coil through which a current which is induced by a vertical component perpendicular to the surface of the position determining plate of the magnetic flux produced, in response to the current flowing through the conductors on the position determining plate. The amount of the vertical component of the magnetic flux detected by the magnetic flux detecting means varies in response to the distance to the conductor, through which the current is flowing, from the magnetic flux detecting means, and when the current flows through the conductors, every one in turn, the ouput signal obtained from the magnetic flux detecting means has its level varying in stepwise manner and varying to be positive from zero, then to be negative from positive through zero, and then to be zero from negative. In such case, the time having elapsed from an instant at which the conductor at the starting end of the position determining plate is supplied with the current, to another instant at which the output signal from the magnetic flux detecting means has a zero-crossover point (on the way to be, for example, negative from positive) corresponds to the position of the magnetic flux detecting means on the position determining plate in the direction of arrangement of the conductors. Accordingly, the position of the magnetic flux detecting means on the position determining plate in the direction of arrangement of the conductors is derived by measuring this time.
The coordinate position digitizing system as mentioned above is used with a cathode ray tube for displaying figures and the position determining plate thereof is placed at a location close to the cathode ray tub. Therefore, the position determining plate is affected by the deflection magnetic field provided to the cathode ray tube to cause horizontal and vertical electron beam scanning therein, and the magnetic flux detecting means detects not only the magnetic flux produced by the current flowing through the conductors on the position determining plate but also the magnetic flux of the deflection magnetic field. Consequently, the output signal from the magnetic flux detecting means contains a noise component resulting from the magnetic flux of the deflection magnetic field so as not to vary in such a manner as mentioned above during the term in which the constant current flows through the conductors one after another, and so the zero-crossover point does not correspond in practice to the actual position of the magnetic flux detecting means on the position determining plate in the direction of arrangement of the conductors. Since the magnetic flux of the deflection magnetic field varies rapidly during a horizontal blinking period and slowly and linearly during a horizontal scanning period in respect of the electron beam scanning in the cathode ray tube, the noise component contained in the output signal from the magnetic flux detecting means has a high frequency during the horizontal blanking period and a low frequency during the horizontal scanning period. As a result of this, erroneous positional data for digitizing a position on the position determining plate would be obtained by merely measuring the time having elapsed as mentioned above due to the noise component contained in the output signal from the magnetic flux detecting means.